library(rgl)

pInt <- seq(0,100,1)
nInt <- seq(0,100,1)
r <- NULL
for(p in pInt){
  for(n in nInt){
    if((p + n) > 0){
      x <- (n / (p + n)) * n
      v <- (1 - exp(-2 * x)) / (1 + exp(-2 * x))
      r <-rbind(r, c(p, n, v))
    }
  }
}

plot3d(r[,1],r[,2],r[,3])